Many conductive inks include a particulate metal, such as silver or aluminum, in a binder or binding medium. While such inks produce conductors (when cured) which are substantially conductive and have a comparatively low electrical impedance (or resistance), the resulting conductors are substantially opaque and do not allow the transmission of any appreciable amount of light in the visual spectrum or other important spectra, such as ultraviolet and infrared spectra.
Optically transparent conductors are needed in a wide variety of applications, however. For example, optically transparent conductors are highly desirable for making electrical contacts to diodes in photovoltaic and in light emitting applications, to allow greater light input and light output, respectively, compared to opaque conductors.
Typical printable transparent conductors, while having reasonable optical transmissivity, unfortunately often have a comparatively high electrical impedance and low conductivity when cured, with resistances typically in the range of 800-1000 or more ohms per square (e.g., polyethylene-dioxithiophene). In addition, many such transparent conductors (e.g., indium tin oxide (ITO)) require specialized deposition techniques and very high temperature processing to reduce impedance, or when cured in a resulting apparatus, tend to have limited, if any, flexibility. The inks or polymers to produce such typical transparent conductors include, for example, polyethylene-dioxithiophene (e.g., “Orgacon” from AGFA Corp. of Ridgefield Park, N.J., USA), a combination of poly-3,4-ethylenedioxythiophene and polystyrenesulfonic acid (marketed as Baytron P and available from Bayer AG of Leverkusen, Germany), a polyaniline or polypyrrole polymer, carbon nanotubes (CNTs), and/or antimony tin oxide (ATO) (with the CNTs, ATO or others typically suspended as particles in any of the various binders, polymers or carriers).
Other printable transparent conductors require significant additional processing after printing. For example, some are created as separate unitary sheets or films which must be laminated onto a substrate, and then subsequently patterned to form the desired, electrically isolated conductors having specific electrical connections, such as through an etching process. Other printable transparent conductors also require significant additional processing following deposition, such as acid washing followed by significant physical compression in nip rollers, for example, in order to create conductive connections among metallic nanowires forming the conductors. Other printable transparent conductors are fragile when deposited, and may further require additional stabilization layers to hold the deposited but unstable metallic nanowires in place. These types of printable transparent conductors have limited usefulness, however, as they cannot be readily utilized to provide electrical connections to devices, such as diodes, which are already placed on a substrate and which should not be subjected to potentially irreparably damaging treatments such as acid washes, etching, or compressive forces, for example.
Accordingly, a need remains for a conductive ink, polymer or composition which may be printed and, when cured, produces a resulting conductor which is stable, fixed in place, and capable of providing electrical connections to devices, and further provides a comparatively low electrical impedance (or resistance) while simultaneously allowing substantial light transmission in the visual or other spectra. In addition, a need remains for such a composition to be capable of curing into a stable conductor at comparatively lower processing temperatures, and be suitable for a wide variety of applications, such as for use in lighting and photovoltaic panels.